International patent application WO 97/29594 describes a method and a device for post-processing decoded video data so as to minimize the blocking artifacts in an image without affecting the contrast.
To this end, the method of post-processing data in accordance with the prior art, described in FIG. 1, comprises the steps of:                LPF filtering (11) decoded video data (x), suitable for delivering filtered data (xf),        DCT transform (12,13) of the filtered data and the decoded video data, suitable for delivering transformed filtered data (Xf) and transformed decoded data (X),        adjusting ADJ (14) the low-frequency coefficients comprised in the transformed filtered data, suitable for delivering adjusted low-frequency coefficients (Xbf),        combining (16) the high-frequency coefficients (Xhf) comprised in the transformed decoded data and the adjusted low-frequency coefficients, suitable for delivering combined transformed data (Xc), and        IDCT transform (17) of the combined transformed data, suitable for delivering processed data (xc) that are ready for display on a screen.        
This method of post-processing decoded video data necessarily comprises a low-pass filtering step for filtering the high-frequency components. This method allows extraction, in the spatial domain, of the low-frequency data on which a first DCT transform and an adjustment are applied in order to deliver adjusted low-frequency coefficients. The high-frequency coefficients are extracted (15) by way of a second DCT transform of the decoded video data. Thus, one part of the data-processing method is realized in the spatial domain, which renders the method both complex to be carried out and costly in terms of computing.
Moreover, a low-frequency coefficient comprised in the transformed filtered data is adjusted within the interval [Xq−q/2,Xq+q/2], where Xq is the value of said quantized low-frequency coefficient and q is the value of the quantization step, so that such a method necessitates access to the encoding parameters, which is not always possible.